In optical communications networks, optical transceiver modules are used to transmit and receive optical signals over optical fibers. A transceiver module generates amplitude and/or phase and/or polarization modulated optical signals that represent data, which are then transmitted over an optical fiber coupled to the transceiver. The transceiver module includes a transmitter side and a receiver side. On the transmitter side, a laser light source generates laser light and an optical coupling system receives the laser light and optically couples, or images, the light onto an end of an optical fiber. The laser light source typically comprises one or more laser diodes that generate light of a particular wavelength or wavelength range. The optical coupling system typically includes one or more reflective elements, one or more refractive elements and/or one or more diffractive elements.
In high-speed data communications networks (e.g., 10 Gigabits per second (Gb/s) and higher), multimode optical fibers are often used. In such networks, certain link performance characteristics, such as the link transmission distance, for example, are dependent on properties of the laser light source and on the design of the optical coupling system. Among the most dominant ones are the modal bandwidth of the fiber and the relative intensity noise (RIN) of the laser light source, which can be degraded by the optical back-reflection to the laser light source. Both of these parameters can be affected by the launch conditions of the laser light into the end of the multimode optical fiber.
In the last decade, extensive investigations have been conducted to determine the effects on modal bandwidth that result when laser diode light sources are used versus when light emitting diode (LED) light sources are used. Based on these investigations, it has been determined that the effective modal bandwidth of multimode fiber is dependent upon the launch conditions of the laser light into the end of the fiber. The launch conditions are, in turn, dependent upon the properties of the laser diode itself and upon the optical coupling system design and configuration. However, due to limitations on the manufacturability of optical elements that are typically used in imaging-type optical coupling systems, control of the launch conditions is limited primarily to designing and configuring the optical coupling system to control the manner in which it images the light from the laser source onto the end of the fiber. Other types of non-imaging optical coupling system designs exist, such as spiral launch designs, for example, that overcome certain disadvantages of the imaging-type optical coupling systems. Such non-imaging systems, however, also have shortcomings.
It is well known that center and edge defects exist in the refractive index profile of multimode fibers. Such defects are generally due to the nature of the processes that are used to manufacture the fibers. It is also known that when these types of fibers are used with laser light sources, the existence of the defects can dramatically change the effective modal bandwidth of the fiber and degrade it below the out-of-factory minimum specification. For these reasons, efforts are made to control the launch conditions of the laser light to prevent the laser light from passing through the areas in the fiber where the defects are most severe and where the occurrence of the defects is more frequent.
For example, in spiral launch optical coupling systems, the laser light from the source is encoded with a phase pattern that rotates the phase of the light linearly around the optical axis of a collimating lens that is used to couple the light from the source onto the end of the optical fiber. Rotating the phase of the laser light about the optical axis helps ensure that refractive index defects in the center of the fiber are avoided. In addition, the spiral launch methodology is relatively successful at reducing optical feedback from the fiber end to the laser aperture, which can destabilize the laser.
The main disadvantage of the spiral launch methodology is that it can result in differential mode coupling efficiency problems. When laser light is launched from a laser having a small numerical aperture (NA), the laser light coupled by the lens onto the fiber end tends to be more spread out. Similarly, when laser light is launched from a laser having a large NA, the laser light coupled by the lens onto the fiber end tends to be more confined. With graded-index multimode fiber, this effect causes unequal coupling efficiencies of the laser light depending on the NAs of the lasers. This inequality in the coupling efficiencies of the different modes can lead to the occurrence of mode-selective noise as well as deterioration of the signal rising edge, which are undesirable.
In many applications, vertical cavity surface emitting laser diodes (VCSELs) are used as the laser light source for generating multimode laser light. In VCSELs, the NAs are smaller for the lower modes and larger for the higher modes. Consequently, the sensitivity of the spiral launch methodology to the NA makes differential mode coupling efficiency an even greater problem in cases where VCSELs are used as the laser light source.
Accordingly, it would be desirable to provide a method and apparatus that enable the launch of the laser light into the end of a fiber to be controlled in such a way that center and edge refractive index defects in the fiber are avoided. It would also be desirable to provide such launch control while maintaining relatively equal coupling efficiencies for different modes and while reducing optical feedback from the fiber end to the laser aperture.